A ladle for receiving molten steel is provided with a sliding nozzle. The ladle provided with the sliding nozzle is filled with fire-resistant sand (also known as filling sand for sliding nozzle or simply as filling sand) before receiving the molten steel poured thereinto so that the molten steel is prevented from coagulating in the nozzle.
Known as the traditional filling sand is mixed sand comprising silica sand and chromite sand (see Patent Document 1: International Publication WO 97/05978). It is relatively unusual for this mixed sand to form a sintered sand layer because of the molten steel and to make non-opening in the nozzle. The mixed sand, however, has become desired nowadays to avoid the non-opening in the nozzle with a probability (hereinafter referred to as a free opening ratio) as close as 100% because the non-opening in the nozzle could cause many problems with continuous casting operations of a facility.